The present invention relates to a focused ion beam (FIB) apparatus.
At present, focused ion beam apparatuses are used in many fields. In particular, a fine focused beam can be applied to a micro area of devices and materials. Furthermore, this apparatus focuses an ion beam from an ion source through a lens and irradiates the beam onto a specimen, and the apparatus is used for milling and observing a specimen in a micro area. For example, it is possible to use the apparatus for observation of a specimen by irradiating an ion beam of a relative low current onto the specimen and detecting secondary particles generated from the specimen. Furthermore, it is possible to mill a specimen by irradiating an ion beam of a relatively high current onto the specimen.
In addition, the focused ion beam is used for structure analysis and failure analysis of microelectrical mechanical systems (MEMS) and semiconductor devices. These elements have been integrated in recent years. In general, therefore, they have a layered structure. For this reason, to inspect them, it is necessary to conduct cross-sectioning up to a specified layer and inspect the cross-section structure. As the MEMS and semiconductor devices become finer, their structures also become gradually complicated. Therefore, the number of cross sections to be inspected increases more and more, and the time which can be used for inspection per cross section becomes shorter and shorter. Since the focused ion beam is applicable to both milling and observation, it is effective to the structure analysis and failure analysis of the MEMS and semiconductor devices. First, the current of the ion beam is increased and milling is conducted from the surface down to a specified layer. Then, the current of the ion beam is decreased and the specified layer is inspected.
For example, a conventional apparatus described in U.S. Pat. No. 5,852,297 has an optical system in which a total optical length from apex of an ion emitter of an ion source to a specimen is in the range of 300 to 450 mm. The distance between the ion emitter and the center of a condenser lens center is 45 mm or less. The distance between the objective lens center and the specimen is 40 mm or less. A FIB formed by this optical system has a maximum current density Jmax of at least 15 A/cm2. As for a fine milling beam in a milling mode, a beam current Ip is Ip≧several tens pA and a beam diameter d is d≦40 nm. As for an observation beam in an observation mode, Ip≧several pA and d≦15 nm.
In the focused ion beam apparatus, the milling position precision substantially depends on the beam diameter d in the milling mode. This results in a problem that, if d is not small enough, the apparatus cannot be suited for the fine structure and features in the structure analysis and failure analysis of the MEMS and semiconductor devices. On the other hand, the milling speed is substantially proportionate to the beam current Ip. If Ip is not large enough, the milling speed becomes slow. For meeting the needs of still higher throughput of the FIB milling in the structure analysis and failure analysis, therefore, it has become a subject to increase the beam current density, i.e., increase the beam current without making the beam diameter of the milling beam large. In the observation mode, on the other hand, the focused ion beam apparatus has a problem that, if the ion beam is not fine enough, it is not possible to observe the structure and features of fine MEMS and semiconductor devices in the structure analysis and failure analysis. Another problem in the observation mode is that, if the current of the ion beam is not large enough, the obtained signal is too small to detect an image with a sufficient signal-to-noise ratio. As semiconductors become finer, it has become a subject to make the observation beam still finer.
The above-described conventional apparatus example does not meet the needs of still higher throughput in FIB milling especially on the high current side. For example, in 30-kV Ga-FIB in the conventional apparatus, a beam having d≈1 μm is associated with Ip≈16 nA. It is now supposed that box milling with a length 20 μm, a width 20 μm and a depth 20 μm is conducted on the surface of a Si specimen by using this beam. The milling yield Y of a Si specimen using 30 kV Ga-FIB depends on the scanning velocity of the FIB, and it is the range of 0.2 to 0.8 μm3/nA·s. Under the condition Y=0.25 μm3/nA·s, the time taken for box milling amounts to approximately 17 minutes. Therefore, it is not possible to meet the needs of higher throughput in the structure analysis and failure analysis of the MEMS and semiconductor devices for conducting a lot of milling of this kind in a short time.